This invention relates to a method for manfacturing cellulosic sheet materials and, more particularly, to a high speed method for forming webs from airlaid wood fibers, the webs being characterized by desirable combination of strength, absorbency, and tactile properties and suitable for uses such as, for example, sanitary wipes and toweling.
Conventionally, materials suitable for use as disposable tissue and towel products have been formed on paper-making equipment by water laying a wood pulp fibrous sheet. Conceptually, this equipment is designed so that the configuration of the resulting sheet approaches a two-dimensional structure. This allows continuous operation at high speeds, and such sheets may be formed at speeds of 3,000 to 4,000 feet per minute. Indeed, recent developments have allowed sustained production at speeds of up to 5,000 feet per minute.
Following formation of the sheet, the water is removed either by drying or by a combination of pressing and drying. As water is removed during formation, surface tension forces of very great magnitude develop which press the fibers into contact, resulting in overall hydrogen bonding at substantially all the intersections of the fibers; and a thin, essentially two-dimensional sheet is formed. It is the hydrogen bonds between fibers which provide the sheet strength, and such bonds are produced even in the absence of extensive additional pressing. Due to this overall bonding phenomenon, cellulosic sheets prepared by water-laid methods inherently possess very unfavorable tactile properties (e.g. harshness, stiffness, low bulk, and poor overall softness) and absorbency for use as sanitary wipes and toweling.
To improve these unfavorable properties, water-laid sheets are typically creped from the dryer roll; i.e. -- the paper is scraped from a dryer roll with a doctor blade. Creping reforms the flat sheet into a corrugated-like structure thereby increasing its bulk and simultaneously breaking a significant portion of the fiber bonds, thus artificially improving the tactile and absorbency properties of the material. But creping raises several problems. It is only effective on low, e.g. -- less than about 15 pounds per 2880 ft..sup.2, basis weight webs, and higher basis weight webs remain quite stiff even after creping and are generally unsatisfactory for uses such as quality facial tissues. Because of this, it is conventional practice to employ at least two plies of creped low basis weight paper sheets for such uses. Only by doing this can a sufficiently bulky product with acceptable softness be prepared. Still further, the detrimental effects of the initial overall bonding in a water-laid paper sheet are not completely overcome.
Sanford et al. (U.S. Pat. No. 3,301,246) proposed to improve the tactile properties of water-laid sheets by thermally pre-drying a sheet to a fiber consistency substantially in excess of that normally applied to the dryer surface of a paper machine and then imprinting the partially dryed sheet with a knuckle pattern of an imprinting fabric. The sheet is thereafter dryed without disturbing the imprinted knuckle-pattern bonds. While this method may somewhat improve the softness, bulk and absorbency of the resulting sheet, the spaces between the knuckle bonds are still appreciably compacted by the surface tension forces developed during water removal and considerable fiber bonding occurs. Creping is still essential in order to realize the maximum advantage of the proposed process; and, for many uses, two plies are still necessary.
As will be apparent from the foregoing discussion, conventional paper-making methods utilizing water are geared towards the high speed formation of essentially two-dimensional sheets which inherently possess the inefficient attribute of initial "overbonding," which then necessitates a creping step to partially "debond" the sheet to enhance the tactile properties.
Air forming of wood pulp fibrous webs has been carried out for many years; however, the resulting webs have been used for applications where either little strength is required, such as for absorbent products, i.e. -- pads, or applications where a certain minimum strength is required but the tactile and absorbency properties are unimportant; i.e. -- various specialty papers. U.S. Pat. Nos. 2,447,161 to Coghill and 2,810,940 to Mills and British Patent No. 1,088,991 illustrate air-forming techniques for such applications.
Indeed, heretofore, it has not been believed that air forming techniques could be advantageously used to prepare cellulosic sheet material that would be sufficiently thin and yet have adequate strength, together with softness and absorbency to serve in applications such as sanitary wipes and toweling.
U.S. Pat. No. 3,692,622 assigned to the same asignee as the present application discloses an aesthetically pleasing web having a combination of strength, absorbency and tactile properties suitable for such applications. This novel product is made by air forming wood pulp fibers to provide a three-dimensional continuum of such fibers and thereafter pattern bonding the fibers by applying a limited amount of moisture and high pressure in a spaced pattern of small areas in the web.
While the properties of such webs make them highly desirable for applications such as sanitary wipes and toweling, widespread usage is contingent upon the ability to make the webs in an economic fashion. This necessitates forming the product at high speeds, desirably in excess of 1,000 feet per minute. Operation at such speeds, however, creates a considerable variety of problems. A particularly difficult problem arises because the airlaid continuum which is formed has little integrity and yet must be subjected to a bonding step that requires removal of the flimsy continuum from the wire on which it was formed. Bonding, at such speeds, to provide the desired strength levels and coherency of the product also creates difficulties.
It is accordingly an object of the present invention to provide a method for forming airlaid cellulosic webs at high speeds.
Another object of the present invention lies in the provision of a high speed method in which the airlaid continuum may be readily transferred from the surface on which it is formed into and out of a bonding station. A related and more specific object is to provide a method of the herein-described type wherein a transfer surface receives the continuum which adheres thereto until completion of the bonding step.